What can I plant in full shade under trees for a border, as my house doesn't have a side fence?
Certified GKH Gardening Expert
Here are a few options: https://www.gardeningknowhow.com/ornamental/shrubs/shgen/shade-loving-shrubs.htm Most won't provide thick privacy cover, but would serve as a property line marker.
To be honest, shrubs directly under trees, even shrubs that like shade, have a hard time. Trees tend to suck out all the available moisture in the soil, which makes it hard for shrubs to survive.